The increasing depletion of world wide energy reserves has prompted a renewed interest and emphasis on developing efficient, high speed railway systems for rapid transit and freight shipment. One phase of research and development in this area has been directed toward devising a rail mounting system which would minimize routine maintenance while providing improved vibration isolation, noise reduction and electrical insulation. In many of the more modern rail systems in the United States, particularly for rapid transit, the rails are mounted directly to concrete supporting structures. The disadvantages of wooden ties or wooden ties embedded in concrete are well known. Replacement of wooden ties embedded in concrete is difficult and time consuming, and in subway systems for example, the additional vertical height required to accommodate the thickness of wooden ties results in added expense for tunnel construction.
While concrete ties generally involve less maintenance and last longer than wooden ties, so-called direct fixation systems for mounting the rails to concrete supporting structures must provide means to absorb the impact between the rail and concrete, to avoid damage to the concrete and provide energy absorption to dampen vibrations and attenuate noise. One of the most common direct fixation rail mounting systems presently in use includes a base assembly consisting of a layar of resilient energy absorbing elastomer disposed between and attaching to a base plate mounted to the concrete and a top plate which supports the rail. Such base assemblies have proved to be reasonably effective in minimizing deterioration of the concrete supporting structure, and reducing vibration and noise transmission to the railway cars and surrounding areas.
However, it has been found that even where the best installation procedures are used, some degree of lateral adjustment must be included in such base assembly designs to assure that the proper gauge is maintained between rails. This is particularly important in areas where the concrete supporting structures are susceptible to sinking, earthquakes or other causes of shifting. U.S. Pat. Nos. 4,062,490 and 3,868,804 to Hixson, for example, disclose base assembly structures in which the position of the rail may be laterally adjusted at the point of anchorage of the base assembly to the concrete. Threaded bolts are embedded in the concrete on opposite sides of the rail, and extend upwardly through corresponding elongated slots formed in the base plate of the base assembly. Fixed brackets or clips attaching to the top plate contact the rail and hold it securely to the base assembly. The entire base assembly including the rail is thus laterally moveable along the elongated slots and secured in the desired position with nuts tightened on to the threaded anchorage bolts.
An alternative approach is found in U.S. Pat. Nos. 3,576,293 and 3,784,097 to Landis in which the base assembly is held in a fixed position relative to the concrete supporting structure, and the brackets or clips which secure the rail are laterally adjustable along the top plate of the base assembly. As in the Hixson disclosures, threaded bolts and nuts are utilized by the Landis systems at the point of anchorage to the concrete and at the point of attachment of the rail to the top plate of the base assembly.
A problem associated with the systems of both Hixson and Landis is that over a period of time the repeated pounding of the rails against the concrete may cause loosening of the threaded bolt-nut attachments both at the anchorage points and at the rail. Due to the lateral adjustment capability in each of the prior art designs mentioned above, loosening of such attachments could result in lateral movement of the rails even in continuously welded track. This can be particularly dangerous in curves where lateral forces tending to urge the rails apart are most prevalent. Thus, continuous maintenance programs are required with such prior art systems to assure that all bolted connections remain tight. In addition, each of the patents cited above include at least one threaded bolt which extends upwardly from the concrete supporting structure and/or at the point of attachment of a bracket or clip to the rail such that the threads are exposed to the weather. As time passes, oxidation and other deterioration of the threads could present maintenance problems in loosening such attachments for lateral adjustment of the rail, or in simply tightening the nuts to the bolts as they become loosened.
In an effort to avoid the potentially substantial costs required for proper maintenance of the systems described above, resilient, one-piece metal clips including so-called Pandrol clips have been utilized as a durable, relatively maintenance-free alternative. See U.S. Pat. No. 3,910,493. The Pandrol rail clip for example, is a resilient metal bar which is bent or formed in a curved shape such that one section contacts the flange of a rail and a second section is secured to the top plate of the base assembly by some form of attachment means. Once in place, the Pandrol clip needs no adjustment, tightening or other form of maintenance unless a failure should occur requiring replacement.
At least one prior art U.S. Pat. No. 4,047,663 to Reynolds et al, has recognized the advantage of using Pandrol clips in combination with the general configuration of standard base assemblies now commonly is use as discussed above. The Reynolds et at direct rail fixation system utilizes eccentrics at the point of anchorage between the base assembly and concrete supporting structure to provide lateral adjustement of the rail. The anchor bolt extending through the eccentric is inserted into a correspondingly threaded sleeve embedded in the concrete. The top plate of the base assembly is formed with generally circular notches or grooves to receive the Pandrol clip and lock it firmly into position against the rail. A disadvantage of this configuration, however, is the same as that described above in connection with the Hixson systems. If the eccentric anchorage connection should loosen after a period of time, the entire base assembly would be susceptible to shifting in response to lateral forces.